Heat a Dutch oven over medium heat. Add oil to pan; swirl to coat. Add mushrooms and the next 4 ingredients (through lelmongrass); cook 3 minutes, stirring occasionally. Add chili paste; cook 1 minute. Add chicken stock, coconut milk, fish sauce, and sugar; bring to a simmer. Reduce heat to low; simmer for 10 minutes. Add chicken to pan; cook 1 minute or until thoroughly heated. Discard lemongrass. Top with onions, cilantro, and lemon juice.